You can create an Application Launcher for composing mail, without opening Gmail (or any other mail). To do this, do the following:
1. Right click on the desktop, and select Create Launcher....
2. You can also create a new item in Applications.
i) Right-click 'Applications', and select Edit Menus.
ii) Select an appropriate menu (Internet in my case) or create a new menu by clicking New Menu, although this isn't necessary. Create an item by clicking New Item.
3. Write firefox mailto:// in the field Command. Give an appropriate name and add a comment if required.
4. Drag the launcher to any of the panels, if required, for the ease of launching.
5. Click the launcher to see your mail editor open up.
You can now compose and send mails.